1. Field of Invention
The present invention relates to the field of separation of particulate matter from liquids, in particular dielectrophoretic separation from liquids having high electrical resistivity such as oil.
2. Description of Related Art
In the industrial world, the separations of impurities in the form of solid from a mixture of such impurities and carrying liquid are of great importance for various purposes. Conventionally, five most commonly used categories of solid-liquid separation methods in such applications have been gravity sedimentation, mechanical filtration separation, centrifugal separation, electrostatic separation, and chemical-assisted separation. The effectiveness of any separation method is closely related to the size of solid particles that are being separated and the physical and chemical properties of the liquid-solid mixture from which they are being separated. The effectiveness of solid-liquid separation directly impacts the cost of manufacturing and environment, so the choice of a suitable separation method is normally based on considerations of technological feasibility, cost and benefit evaluation, and environmental impact.
Crude oil has played a significant role in today's world, and its processed products like gasoline, diesel are vital for today's industrial world and people's life. The quality of the crude oil refining products, like the particulates and vanadium compounds in jet fuel, is also crucial for the healthy operation and lifetime of jet engines.
Furthermore, burning fuels with contaminants will cause server environmental pollution. In developed countries like US and Britain, fuel oils are required to meet the approval standards with regards to solid contaminants concentrations, i.e. 120 ppm. Therefore, there is a need for effective yet low operation cost technology to meet this application need. It is an objective of the invention to enhance the particle removal efficiency required by the oil refinery industry.
One of the most challenging applications is to remove spent catalyst particles from the heavy cycle oil and slurry oil by-product of Fluid Catalyst Cracking (FCC), which is widely used to convert the high-boiling, high-molecular weight hydrocarbon fractions of petroleum crude oils to smaller chains comprising more valuable gasoline, olefinic gases, and other products. The feedstock to an FCC is usual that portion of the crude oil that has an initial boiling point of 340° C. or higher at atmospheric pressure and an average molecular weight ranging from about 200 to 600 or higher. The FCC process vaporizes and breaks the long-chain molecules of the high-boiling hydrocarbon oils into much shorter molecules by contacting the feedstock, at high temperature and moderate pressure, with a fluidized powdered catalyst.
The catalyst is a solid sand-like material that is made fluid by the hot vapor and oil fed into the FCC. In general, the spent catalyst particles' sizes range from 0.5˜80 micrometer, with majority of them under 10 microns. The most common kind of FCC catalyst is a solid sand-like fine powder with a bulk density of 0.80 to 0.96 g/cm3 that is made fluid by the hot vapor and oil fed into the FCC. The fresh FCC catalyst fines normally have a particle size distribution ranging from 10 to 150 μm and an average particle size of 50 to 100 μm. The design and operation of an FCC unit is largely dependent upon the chemical and physical properties of the catalyst. By contacting the fluidized catalyst powders, the majority of the long-chain FCC hydrocarbon feedstock breaks the long-chain molecules into much shorter molecules, while a small percentage portion (2%-9%) of the long-chain hydrocarbon feedstock oil, called FCC slurry oil or decant oil, will be unbreakable and reside at the bottom of the FCC processing fractionator. The FCC residual oil or slurry oil contains a high concentration of FCC catalyst ranging from 1000 ppm to 10,000 ppm.
The spent catalyst particles, while useful for “cracking” and reducing the length of the hydrocarbons, are harmful for the processes that transform slurry oil into raw material for more value added applications. Traditionally, the slurry oil containing the catalysts is pumped through a slurry settler, wherein, after settling, the bottom oil contains most of the catalyst particles and is recycled into the feedstock, or in clarified form, for use as heavy fuel oil. Failure to remove the spent catalyst particles often results in the slurry oil being used as a lesser grade heavy fuel for cargo fleets, which in addition to being less efficient, aggregate air pollution.
Particles with diameter smaller than 20 microns take a long time, usually in the range of days, to sediment, which makes the gravity sedimentation method impractical for refineries to adopt in a mass-production environment. Mechanical filtration is ineffective for such small-sized particles because the metal mesh and powder sintered filters have poor uniformity of pore size, and the particle concentration in the filtered slurry oil still in the range of hundreds of ppm. Electrostatic separation methods that have been adopted in the industry, however, have demonstrated high particle removal efficiency. Drawbacks of electrostatic separation include a high operating cost, a process that is highly selective in terms of slurry oil feedstock's physical properties, and an unstable performance. Moreover, electrostatic separation is ineffective when the particle concentration in the slurry oil is over 6000 ppm.
Centrifugal separation can yield good particle removal efficiency unselective to physical properties of slurry oil, but the slurry oil needs to be pre-heated to over 200° C. resulting in a high operation cost, a long processing cycle and low processing capacity. Since a high rotational energy is needed for the entire liquid media to separate 0.5% or 5000 ppm, the energy efficiency is low and only practical for high value materials like nuclear reactor fuel. For a typical refinery having 100 kilotons/year in slurry oil, the centrifugal separation is highly cost-prohibitive and impractical due to a high temperature requirement.
Most mechanical filters will accumulate a filtration cake to achieve the filtration efficiency for the micro-particles under 10 μm in size. As a result, mechanical filters are quickly clogged up during normal operation, and need to be cleaned or backwashed regularly once the pressure drop reaches a limit. In practice it is very difficult to completely clean the plugged micrometer holes. The clogged residual particles will build up in or around the opening, and the pressure drop through filter will increase, limiting the filtration capacity. Once a clogged filter can no longer be cleaned, it must be replaced. Most of refineries with highly viscous slurry oil have found that this method is cost-prohibitive due to the need to regularly replace metal filtration cores.
Starting in 1970s, a new type of filter was developed, called “Electrofilter” or “Electrostatic Separator”, which featured a glass bead bed or porous filtration media under high electric fields from the high voltage (up to 50 kVDC) electrodes. Several types of electrofilters were invented and developed to remove metal compound solids or catalyst fines from the heavy oil for last two decades. Electrofilter technology advanced to fluidize the glass bead bed, which consisted of spherical smooth surfaces, to overcome the cleaning difficulties experienced during the mechanical filter operation. This cleaning process is more effective when the electric field is turned off. Even though there are few scientific references or theoretical studies directly focused on the electrofilter's electrostatic separation mechanisms, the electrofilter achieved some degree of success in refineries.
In the past, dielectrophoretic separation was applied in biophysics or biomedical fields for cell separation. The name Dielectrophoresis “DEP” was first adopted by Pohl in 1950s for the unique electromechanics of particles suspended in a fluid medium when exposed to an applied electric field gradient. In a uniform electric field, the field-induced force on charge-neutral dielectric particles is zero or infinitesimal small. Once the applied electric field is non-uniform or has a gradient, the difference in dielectric polarization between the particles and the fluid medium result a net force on the polarized particles, called “dieletrophoretic forces” (DEP forces). The DEP formula can be derived and quantified in terms of the effective electromagnetic dipole forces on the polarized particles induced by the applied electrostatic field. For the simple case of a spherical particle of radius R and permittivity εp immersed in a lossless dielectric fluid of permittivity εm and subject a non-uniform electric field E:FDEP=2πR3εmκ(∇E2),  (1)Where k is (εp−εm)/(εp+2εm), the real part of Clasius-Mossotti factor, that represents the effective polarizability of the particle with respect to the liquid medium. The (∇E2) quantifies the electric field strength and gradient. Equation (1) indicates that DEP force is proportional to the volume or size of the particle, and the strength and gradient of the applied electric field E. Accordingly, DEP filtration systems can be designed and improved by designing the effective gradient and strength region for the applied electric field.
However, there have been problems applying DEP principles to resolve the industrial solid-liquid separation or filtration issues, particularly the spent catalyst removal issue from FCC slurry oil. In a high-pressure and high-temperature operating environment, refining processes commonly adopt cylindrical vessels as the reactor chambers. A gradient electric field between the center and the side of the vessel, through which a fluid passes, is normally induced by an isolated central high voltage electrode. Under a 3000V high voltage central-electrode 5 cm-cylindrical electrostatic field, a 5 μm particle with a density 2 g/cm3 in the oil, will experience a DEP force of about 2×10−13 N, which is about 25 times weaker than the gravitational force due to the particle's weight 5×10−12 N. It is impractical to apply more than 50 k VDC to a 5 cm cylinder in industrial applications. Today, there remain few industrial DEP applications, and one example is solid-liquid separations. So far, DEP is mostly employed in trapping or separating particles, such as blood cells and cancer cells, in biological or biomedical applications, where a DEP force 10 times stronger than a gravitational settling force can be created by applying micro-electromechanical structural electrodes under normal 10 s VDC or VAC.
In last decade, thousands of technical publications and hundreds of patents were created on the microscopic DEP biomedical applications, such as cell separation and trapping. In order to apply DEP into the industrial applications, such as removing tons of catalyst fines from thousands of tons of the refining process oil streams, like slurry oil, the stronger DEP forces on the particulates must be created to achieve practical efficiency and processing capacities. Therefore there is a need for a device and process to produce these stronger DEP forces in a consistent manner to facilitate solid-liquid separation in oil applications.